Photovoltaic devices utilize specific conductivity characteristics of materials generally referred to as semiconductors, whereby solar energy or radiation is converted to useful electrical energy. This conversion results from the absorption of photon energy in the active region of the cell, whereby some of the absorbed energy causes the generation of electron-hole pairs. The energy required for the generation of electron-hole pairs in a semiconductor material is referred to as the band gap energy and generally is the minimum energy required to excite an electron from the valence band to the conduction band.
There are two principal measures of the utility of photovoltaic devices. First is the efficiency of the device, which is an ascertainable percentage of the total photon energy converted to useful electrical energy. High efficiency photovoltaic devices made of crystalline materials maximize efficiency by minimizing internal lattice defects. The second measure of the utility of a photovoltaic device is its cost. Single crystal devices are complex and costly to produce, and do not readily lend themselves to high volume production.
One approach to reducing the cost of photovoltaic devices is to utilize polycrystalline thin film materials and a heterojunction. A heterojunction refers to the active junction formed at the interface between two dissimilar materials, such as cadmium sulfide and cadmium telluride, as taught by Basol, et al. in U.S. Pat. No. 4,388,483. Basol, et al. describe thin-film heterojunction photovoltaic cells wherein the active layer comprises at least one of the metal elements of Class IIB of the Periodic Table of Elements and one of the non-metal elements of Class VIA of the Periodic Table of Elements. One feature of such photovoltaic devices is the use of extremely thin-film type active layers. As an example, Basol, et al. utilized a cadmium sulfide layer on the order of 0.02-0.05 micrometer and a cadmium telluride layer on the order of about 1.3 micrometers. While such economy of material has obvious advantages, it has also presented an unexpected concern with respect to current collection.
Such thin-film photovoltaic devices typically comprise an optically transparent substrate through which radiant energy enters the device, a first semiconductor layer formed on the transparent substrate, a second semiconductor layer of different conductivity type than the first semiconductor layer and forming a junction with the first layer, and a conductive film back contact. When the substrate is not electrically conductive, a transparent electrically conductive film is disposed between the substrate and the first semiconductor layer to function as a front contact current collector; this front contact generally being a layer of a transparent conductive oxide. Transparent conductive oxides, such as indium tin oxide, indium oxide, zinc oxide, and tin oxide are not efficient current collectors in cells of any appreciable size, that is greater than about one square centimeter, due to their inherent resistivity, which is on the order of about 10 ohms per square at thicknesses necessary for good optical transmission. The transparent conductive layer must be supplemented with more efficient current collection means such as a front contact current collector grid. This grid comprises a network of very low resistivity material that collects electrical current from the transparent conductive layer and efficiently channels the current to a central current collector.
Front contact current collector grids are generally made of materials such as copper, gold, and silver. Since the grid material is not optically transparent, the presence of the grid will lower the overall efficiency of the photovoltaic device. To minimize this disadvantage, current collector grids are designed to cover as little active surface area as possible. One way in which this is done is by forming extremely thin grid lines in relation to the active surface area of the photovoltaic device. When the grid comprises such narrow strips, then the height of each grid line may be calculated, based on the grid material, as a function of resistivity. To obtain a desired low resistivity, the height of the gridline is often found to exceed the thickness of the first active semiconductor layer and to extend into the second active semiconductor layer. This geometry creates problems with respect to shorting of the device and to uniform formation of the semiconductor layers themselves.
Generally, a front contact current collector grid is deposited onto a transparent, conductive layer and followed by subsequent depositions of the active semiconductor layers. This procedure has several drawbacks when applied to the formation of a front contact current collector grid for a photovoltaic device when the thickness of the grid exceeds the thickness of the first semiconductor layer.
A first concern relates to diffusion of the grid material into the semiconductor layers. If subsequent process steps for the formation of the semiconductor layers include treatments at elevated temperatures that encourage diffusion, or if the semiconductor layers are electrodeposited in a bath that may dissolve the grid material, then both the grid and the semiconductor layers may be adversely affected. In the latter instance, the grid may be deteriorated and the bath may become contaminated with leached grid material, which then appears in the deposited semiconductor layer.
A second concern arises with regard to the deposition of semiconductor layers subsequent to the formation of the current collector grid. The deposition may not be uniform or complete due to the presence of the current collector grid, which has a relatively large thickness. It has been seen that the electrodeposition of semiconductor materials near such a grid is imperfect, often leaving exposed portions of the transparent conductive layer which may become eroded. Under such circumstances, the grid is separated from the semiconductor layers by a large resistance or is completely electrically insulated from the semiconductor layers and cannot be an effective current collector. This has the adverse result of increasing the series resistance of the photovoltaic device, and may ultimately cause failure of such a device.
In addition, front contact current collector grids in thin-film photovoltaic devices where the grid thickness exceeds the combined thicknesses of the semiconductor layers must generally be covered with an insulating material to prevent short circuiting between the current collector grid and a back electrical contact. Such an insulation layer is costly and time-consuming to incorporate into a device.
What is needed in the area of efficient current collection in photovoltaic devices is a front contact current collector grid that does not give rise to the above-described deficiencies nor necessarily require the use of an insulation layer.
It is therefore one object of the present invention to provide a front contact current collector grid for photovoltaic devices, which current collector grid is not accompanied by the above-identified shortcomings.
It is another object of the present invention to provide a photovoltaic device having a stable, front contact current collector grid incorporated therein.
It is still another object of the invention to provide a photovoltaic device that does not require an insulation layer disposed between the front contact current collector grid and the back electrical contact.
It is yet another object of the present invention to provide a method for the formation of a photovoltaic device that has a stable, front contact current collector grid incorporated therein.
These and additional objects of the present invention will become apparent to one skilled in the art from the below description of the invention and the appended claims.